阅读说明：本技术 磁路、法拉第旋光器和磁光学元件 (Magnetic circuit, Faraday rotator, and magneto-optical element ) 是由 铃木太志 小田原峻也 于 2019-05-30 设计创作，主要内容包括：本发明提供一种磁路和具备该磁路的法拉第旋光器及磁光学元件。磁路(1)具有由钐－钴系磁铁构成的分别设置有供光通过的贯通孔的第一～第三磁铁,由第一～第三磁铁在前后方向上依次配置于同轴上而构成,在将光通过磁路的贯通孔的方向设为光轴方向时,第一磁铁(11)以使贯通孔侧成为N极的方式在与光轴方向垂直的方向上被磁化,第二磁铁(12)以使第一磁铁(11)侧成为N极的方式在与光轴方向平行的方向上被磁化,第三磁铁(13)以使贯通孔侧成为S极的方式在与光轴方向垂直的方向上被磁化,第二磁铁具有第一、三磁铁以上的矫顽力。由此,能够抑制外部磁场和温度上升引起的不可逆退磁且能够对法拉第元件稳定地提供充分的磁通密度。(The invention provides a magnetic circuit, and a Faraday rotator and a magneto-optical element provided with the magnetic circuit. The magnetic circuit (1) has first to third magnets each including a samarium-cobalt magnet and provided with a through hole through which light passes, the first to third magnets being coaxially arranged in the front-rear direction, the first magnet (11) being magnetized in a direction perpendicular to the optical axis direction so that the through hole side becomes an N pole, the second magnet (12) being magnetized in a direction parallel to the optical axis direction so that the first magnet (11) side becomes an N pole, the third magnet (13) being magnetized in a direction perpendicular to the optical axis direction so that the through hole side becomes an S pole, the second magnet having a coercive force equal to or larger than that of the first and third magnets. This makes it possible to stably provide a sufficient magnetic flux density to the faraday element while suppressing irreversible demagnetization caused by an external magnetic field and a temperature rise.)

1. A magnetic circuit comprising first to third magnets each of which is composed of a samarium-cobalt magnet and is provided with through holes through which light passes, characterized in that:

The magnetic path is formed by arranging the first to third magnets coaxially in the front-rear direction in order,

The first magnet is magnetized in a direction perpendicular to the optical axis direction so that the through hole side becomes an N-pole when a direction in which light passes through the through hole of the magnetic circuit is defined as the optical axis direction,

The second magnet is magnetized in a direction parallel to the optical axis direction so that the first magnet side becomes an N pole,

The third magnet is magnetized in a direction perpendicular to the optical axis direction so that the through-hole side becomes an S-pole,

The second magnet has a coercive force equal to or higher than that of the first magnet and that of the third magnet.

2. The magnetic circuit of claim 1, wherein:

The first to third magnets have a coercive force of 650kA/m or more.

3. a magnetic circuit according to claim 1 or 2, characterized in that:

The length of the second magnet in the optical axis direction is equal to or greater than the length of the first magnet and the third magnet in the optical axis direction.

4. A magnetic circuit according to any of claims 1-3, wherein:

The second magnet has a length in the optical axis direction that is greater than lengths in the optical axis direction of the first magnet and the third magnet.

5. A magnetic circuit according to any of claims 1-4, wherein:

The second magnet has a larger coercive force than the first magnet and the third magnet.

6. A magnetic circuit according to any of claims 1-5, wherein:

the cross-sectional area of the through-hole is 100mm ² or less.

7. A magnetic circuit comprising first to third magnets each provided with a through hole through which light passes, characterized in that:

the magnetic path is formed by arranging the first to third magnets coaxially in the front-rear direction in order,

The first magnet is magnetized in a direction perpendicular to the optical axis direction so that the through hole side becomes an N-pole when a direction in which light passes through the through hole of the magnetic circuit is defined as the optical axis direction,

The second magnet is magnetized in a direction parallel to the optical axis direction so that the first magnet side becomes an N pole,

The third magnet is magnetized in a direction perpendicular to the optical axis direction so that the through-hole side becomes an S-pole,

The second magnet has a coercive force equal to or higher than that of the first magnet and that of the third magnet,

the length of the second magnet in the optical axis direction is equal to or greater than the length of the first magnet and the third magnet in the optical axis direction.

8. A faraday rotator, comprising:

The magnetic circuit of any of claims 1-7; and

And a faraday element which is disposed in the through hole of the magnetic circuit and is composed of a paramagnetic material that transmits light.

9. A Faraday rotator according to claim 8, characterized in that:

the paramagnetic substance is made of glass material.

10. A magneto-optical element, comprising:

A faraday rotator according to claim 8 or 9; and

A first optical member disposed at one end of the magnetic circuit of the Faraday rotator in the optical axis direction and a second optical member disposed at the other end,

The light passing through the through hole of the magnetic circuit passes through the first optical member and the second optical member.

11. The magneto-optical element of claim 10, wherein:

The first optical component and the second optical component are polarizers.

Technical Field

The invention relates to a magnetic circuit, a Faraday rotator, and a magneto-optical element.

Background

The faraday rotator is an element composed of a faraday element and a magnet for applying a magnetic field to the faraday element. Faraday rotators have a function of propagating light only in one direction and blocking return light, and therefore are used as magneto-optical elements such as optical isolators and the like in laser oscillators such as optical communication systems and laser processing systems.

The wavelength band used in optical communication systems is mainly 1300nm to 1700nm, and in current faraday rotators, rare-earth iron garnet is used for faraday elements.

On the other hand, in recent years, a wavelength band used for laser processing or the like is on a shorter wavelength side than an optical communication band, and about 1000nm is mainly used. In this wavelength band, the rare earth iron garnet cannot be used because of its large light absorption, and thus a paramagnetic crystal such as Terbium Gallium Garnet (TGG) is used for the faraday element.

However, in order to use such a faraday rotator as an optical isolator, the rotation angle (θ) of faraday rotation needs to be 45 °. It is known that the rotation angle has a relationship expressed by the following formula (1) with the length (L) of the faraday element, the verdet constant (V), and the magnetic flux density (B) parallel to the optical axis.

θ＝V·B·L (1)

The verdet constant depends on the characteristics of the material of the faraday element. Generally, since a paramagnetic material such as TGG has a smaller verdet constant than a rare-earth iron garnet, it is necessary to increase the length of the faraday element and the magnetic flux density parallel to the optical axis applied to the faraday element in order to obtain a faraday rotation angle of 45 °. In particular, in recent years, miniaturization of the apparatus has been desired, and therefore, there has been proposed a technique for improving the magnetic flux density applied to the faraday element by designing the structure of the magnet without increasing the size of the faraday element or the magnet.

For example, patent document 1 discloses a faraday rotator including a magnetic circuit including first to third magnets and a faraday element, the first magnet being magnetized in a direction perpendicular to an optical axis and toward the optical axis, the second magnet being magnetized in a direction perpendicular to the optical axis and away from the optical axis, the third magnet being disposed between the first and third magnets, the third magnet being magnetized in a direction parallel to the optical axis and toward the first magnet from the second magnet, and a relationship of L ₂/10 ≦ L ₃ ≦ L ₂ is satisfied when a length of the first and second magnets in the optical axis direction is L ₂ and a length of the first and second magnets in the third optical axis direction is L ₃ in the magnetic circuit.

Disclosure of Invention

Technical problem to be solved by the invention

in the magnetic circuit, a strong magnetic field generated by the interaction between the first magnet and the second magnet occurs in the vicinity of the through hole of the third magnet. The magnetic field is in a direction opposite to the magnetization direction of the third magnet. In this way, when an external magnetic field in the direction opposite to the magnetization direction is generated, it is necessary to consider the influence of the movement of the operating point of the magnet and demagnetization attributed thereto. For the purpose of this description,

Fig. 7 shows an example of demagnetization curves (B-H curve and J-H curve) of a magnet, a B-H curve 32 shows a relationship between a magnetic flux density B and an external magnetic field H, and a J-H curve 33 shows a relationship between magnetization J and an external magnetic field H, and intersections of the curves and the vertical and horizontal axes respectively show a remanent magnetic flux density Br, an intrinsic coercive force H _cJ, and a coercive force H _cB, the remanent magnetic flux density Br is a magnetic flux density remaining in a magnetic body when the external magnetic field is changed from a saturated magnetization state to 0, the coercive force is a magnetic field acting in a direction opposite to the magnetization direction held by the magnetic body, and the value of the external magnetic field when the magnetization or magnetic flux density becomes 0 is expressed by an intrinsic coercive force H _cJ and the latter by a coercive force H _cB, and the known magnetic flux density (B) has a relationship with the following expression (2) with the external magnetic field strength (H), the magnetization strength (J), and the permeability (μ ₀) in vacuum.

B＝μ₀H+J (2)

(irreversible demagnetization by external magnetic field)

As shown in fig. 7, when an external magnetic field H1 is applied to the magnet, the operating point a1 of the magnet moves on the B-H curve 32 and moves to the operating point B1 close to the H axis. In addition, when a larger external magnetic field H2 is supplied, the operating point a1 of the magnet moves to the operating point c1 closer to the H axis. At this time, the operating point c1 on the B-H curve 32 is projected to the operating point c2 on the J-H curve 33 beyond the inflection point 34 (the changing point where the magnetic flux density sharply decreases while changing in a gradient) of the J-H curve 33. In this way, when demagnetization occurs due to a larger external magnetic field H2, the operating point a1 of the magnet moves to the operating point d1 when the external magnetic field H2 is removed by projecting the operating point on the B-H curve 32 to the operating point c2 on the J-H curve 33 beyond the inflection point 34 of the J-H curve 33. Here, the operating point d1 is an operating point when the external magnetic field H2 is removed from the operating point c1, and is an intersection point of a straight line parallel to the inclination of the recoil permeability curve passing through the operating point c1 and a straight line passing through the operating point a1 and the origin. At this time, the difference between the magnetic flux density at the operating point a1 and the magnetic flux density at the operating point d1 is irreversible demagnetization Δ B by the external magnetic field H2, and demagnetization is performed in which recovery is not performed unless demagnetization is performed.

As shown in fig. 7, the operating point of the magnet is a point on a B-H curve 32 of the magnet, and represents the state of the magnetic flux density B and the magnetic field H of the magnet in the magnetic circuit, and a straight line drawn from the origin to this point is referred to as a magnet wire 31, and it is known that the inclination (B/H) of the magnet wire 31 is in the relationship between the permeability (μ ₀) and the permeability (P) in the vacuum and the following expression (3).

B/H＝μ₀P (3)

(irreversible demagnetization by high temperature)

_{cJ cB cJ cB}In this temperature change, when the operating point on the B-H curve exceeds the inflection point 34 of the B-H curve, even if the temperature condition is restored, the magnetic force does not restore, that is, the magnet is caused to undergo a temperature change, for example, when the external temperature changes from low temperature to high temperature, the operating point a1 moves to the operating point B1, when the operating point B84 does not exceed the inflection point 34 of the B-H curve, the operating point B-H curve moves from the operating point B36 to high temperature, the operating point a 968 moves from the operating point B-H curve to the operating point B968, when the external temperature changes from low temperature to high temperature, the operating point B-H curve moves from the operating point B36 to the operating point B38, and when the external temperature changes from high temperature to low temperature, the operating point a 368 moves from the operating point B36 to the operating point B2, the operating point B-H curve 35 moves from the operating point B-H34 to the operating point B-H38, and when the external temperature returns to the operating point B-H38, the operating point B-H curve 38 moves from the operating point B38 to the operating point B11B 36, and the operating point B36, the operating point B-H6B 38 moves from the operating point B36 to the operating point B36, and the operating point B38 moves from the operating point B11B 34.

further, the third magnet described in patent document 1 has a shape shorter than the first and second magnets in the magnetization direction. In such a magnet having a short magnetization direction, the magnetic permeability depending on the shape of the magnet needs to be considered. Generally, the magnetic permeability is reduced in a magnet having a shape in which the magnetization direction is short, that is, the magnetic poles are close to each other. The permeability is also the slope of the magnet wire 31 as shown in formula (3). For example, as shown in fig. 9, the magnet wire 31 passing through the action point a1 has a slope α that is greater than the slope β of the magnet wire 31 passing through the action point b1, i.e., a greater permeability coefficient. In other words, since the magnet having the small permeability β has an operating point at a position close to the H axis, the operating point is more likely to exceed the turning point 34 on the B-H curve 32 and the J-H curve 33, and the above-described inverse magnetic field and irreversible demagnetization caused at high temperature are more likely to occur.

As described above, the third magnet of the magnetic circuit described in patent document 1 is used as a magnet at an unfavorable operating point, and therefore, irreversible demagnetization is likely to occur in a reverse magnetic field and at a high temperature. In particular, when the magnetic circuit is used as the magneto-optical element of the high-output laser isolator lamp, the temperature rise of the magnetic circuit accompanying the temperature rise of the faraday element due to the high-output light cannot be avoided, and therefore, the third magnet is likely to undergo irreversible demagnetization due to a high temperature. When irreversible demagnetization occurs in the third magnet, a sufficient magnetic flux density cannot be stably supplied to the faraday rotator, and thus the faraday rotator may not be able to achieve its original function.

The present invention has been made in view of the above-described problems, and an object of the present invention is to provide a magnetic circuit capable of suppressing irreversible demagnetization caused by an external magnetic field and a temperature rise and stably providing a sufficient magnetic flux density to a faraday element.

means for solving the problems

_cBA magnetic circuit of the present invention includes first to third magnets each including a samarium-cobalt magnet and having a through hole through which light passes, wherein the magnetic circuit is formed by arranging the first to third magnets coaxially in the front-rear direction, and when the direction in which light passes through the through hole of the magnetic circuit is the optical axis direction, the first magnet is magnetized in the direction perpendicular to the optical axis direction so that the through hole side has an N pole, the second magnet is magnetized in the direction parallel to the optical axis direction so that the first magnet side has an N pole, the third magnet is magnetized in the direction perpendicular to the optical axis direction so that the through hole side has an S pole, and the second magnet has a correction force equal to or greater than the first magnet and the third magnet.

In the above configuration, a strong magnetic field is generated in the vicinity of the through hole of the second magnet by the interaction between the first magnet and the third magnet. However, in the magnetic circuit of the present invention, since the coercive force of the second magnet is large, the operating point hardly exceeds the turning point on the B-H curve or the J-H curve, and occurrence of irreversible demagnetization due to temperature rise and a reverse magnetic field can be suppressed, and the magnetic flux density in the through hole portion of the second magnet can be easily maintained largely. That is, a sufficient magnetic flux density can be stably supplied to the faraday element.

the first to third magnets are magnets made of samarium-cobalt magnets. The samarium-cobalt magnet has a residual magnetic flux density and a coercive force equivalent to those of a neodymium magnet, but has characteristics that a change in coercive force due to a temperature change is small and a curie temperature is high. Therefore, in the magnetic circuit including the magnet, the operating point is hard to exceed the turning point on the B-H curve or the J-H curve particularly at high temperature, and the occurrence of irreversible demagnetization can be suppressed, and the magnetic flux density in the through hole portion of the second magnet can be easily maintained to a large extent.

In the magnetic circuit of the present invention, the first to third magnets preferably have a coercive force of 650kA/m or more. By having such a coercive force, the occurrence of irreversible demagnetization in the second magnet can be suppressed.

In the magnetic circuit of the present invention, it is preferable that the length of the second magnet in the optical axis direction is equal to or greater than the length of the first and third magnets in the optical axis direction. Accordingly, it is possible to suppress the occurrence of irreversible demagnetization in the second magnet and easily maintain the magnetic flux density in the through hole portion of the second magnet to a large extent.

In the magnetic circuit of the present invention, it is preferable that the length of the second magnet in the optical axis direction is larger than the lengths of the first and third magnets in the optical axis direction.

In the magnetic circuit of the present invention, the second magnet preferably has a coercive force larger than those of the first and third magnets.

in the magnetic circuit of the present invention, it is preferable that the cross-sectional area of the through-hole is 100mm ² or less, and the magnetic flux density is easily increased by setting the cross-sectional area of the through-hole to 100mm ² or less.

The magnetic circuit of the present invention includes first to third magnets each having a through hole through which light passes, and is characterized in that the magnetic circuit is configured by the first to third magnets being coaxially arranged in the front-rear direction, and when the direction in which light passes through the through hole of the magnetic circuit is the optical axis direction, the first magnet is magnetized in the direction perpendicular to the optical axis direction so that the through hole side becomes the N pole, the second magnet is magnetized in the direction parallel to the optical axis direction so that the first magnet side becomes the N pole, the third magnet is magnetized in the direction perpendicular to the optical axis direction so that the through hole side becomes the S pole, the second magnet has a coercive force equal to or greater than that of the first magnet and the third magnet, and the length of the second magnet in the optical axis direction is equal to or greater than the length of the first magnet and the third magnet in the optical axis direction.

In the above configuration, a strong magnetic field is generated in the vicinity of the through hole of the second magnet by the interaction between the first magnet and the third magnet. However, since the coercive force of the second magnet is large, the operating point hardly exceeds the turning point on the B-H curve or the J-H curve, the occurrence of irreversible demagnetization due to temperature rise and the reverse magnetic field can be suppressed, and the magnetic flux density in the through-hole portion of the second magnet can be easily maintained largely. Further, since the length of the second magnet in the optical axis direction is equal to or greater than the length of the first and third magnets in the optical axis direction, it is possible to suppress the occurrence of irreversible demagnetization in the second magnet and easily maintain the magnetic flux density in the through hole portion of the second magnet to a large extent.

The faraday rotator of the present invention comprises the magnetic circuit and a faraday element which is disposed in the through hole of the magnetic circuit and is composed of a paramagnetic substance that transmits light.

the paramagnetic substance of the faraday rotator of the present invention is preferably a glass material.

The magnetic optical element of the present invention includes the above-described faraday rotator, and a first optical member disposed at one end of the faraday rotator in the optical axis direction of the magnetic circuit and a second optical member disposed at the other end, and light passing through the through hole of the magnetic circuit passes through the first optical member and the second optical member.

In the magneto-optical element according to the present invention, the first optical member and the second optical member are preferably polarizers.

Effects of the invention

According to the present invention, it is possible to provide a magnetic circuit capable of suppressing irreversible demagnetization caused by an external magnetic field and a temperature rise and stably providing a sufficient magnetic flux density to a faraday element.

Drawings

fig. 1 is a schematic cross-sectional view showing an example of the structure of a magnetic circuit according to the present invention.

Fig. 2 is a diagram showing an example of the structure of the first magnet in the present invention.

Fig. 3 is a diagram showing an example of the structure of the second magnet in the present invention.

fig. 4 is a diagram showing an example of the structure of the third magnet according to the present invention.

FIG. 5 is a schematic cross-sectional view showing an example of the structure of a Faraday rotator of the present invention.

Fig. 6 is a schematic cross-sectional view showing an example of the structure of the magneto-optical element of the present invention.

Fig. 7 is a diagram showing an example of demagnetization curves (B-H curve and J-H curve) of the magnet.

Fig. 8 is a diagram showing an example of temperature changes (B-H curve and J-H curve) of the demagnetization curve of a neodymium magnet.

Fig. 9 is a diagram showing an example of demagnetization curves (B-H curve and J-H curve) of the magnet.

Detailed Description

Hereinafter, embodiments of the present invention will be described in detail. However, the present invention is not limited to the following embodiments. In the drawings, components having substantially the same function are sometimes referred to by the same reference numerals.

(magnetic circuit 1)

Fig. 1 is a schematic cross-sectional view showing the structure of a magnetic circuit of the present invention. The magnetic circuit 1 includes a first magnet 11, a second magnet 12, and a third magnet 13 each having a through hole. The magnetic circuit 1 is configured by arranging a first magnet 11, a second magnet 12, and a third magnet 13 coaxially in the front-rear direction in this order. The coaxial arrangement means that the magnets are arranged so as to overlap each other near the center of each magnet when viewed from the optical axis direction. In the present embodiment, the through-holes connecting the first magnet 11, the second magnet 12, and the third magnet 13 constitute the through-hole 2 of the magnetic circuit. Note that the characters N and S in fig. 1 denote magnetic poles, and the same applies to other drawings described later.

In the magnetic circuit 1, the first magnet 11 and the third magnet 13 are magnetized in a direction perpendicular to the optical axis, and the magnetization directions are opposite to each other. Specifically, the first magnet 11 is magnetized in a direction perpendicular to the optical axis so that the through-hole side becomes the N-pole. The third magnet 13 is magnetized in a direction perpendicular to the optical axis so that the through-hole side becomes the S-pole. The second magnet 12 is magnetized in a direction parallel to the optical axis so that the first magnet 11 side becomes an N pole.

The first to third magnets constituting the magnetic circuit 1 are preferably made of a magnet having samarium-cobalt (Sm-Co) as a main component. The samarium-cobalt magnet has a Curie temperature of 600 ℃ or higher, and therefore, irreversible demagnetization at high temperatures can be suppressed. Further, the temperature dependency of the residual magnetic flux density of the samarium-cobalt magnet is generally about-0.03%/deg.C, and the temperature dependency of the neodymium magnet is about-0.1%/deg.C. In addition, the temperature dependence of coercive force is about-0.5%/K in a neodymium magnet and about-0.2%/K in a samarium-cobalt magnet. Therefore, when the samarium-cobalt-based magnet is used, the decrease in the residual magnetic flux density and coercive force of the magnet due to the temperature increase of the magnetic circuit 1 can be more effectively suppressed. In addition, a magnet having samarium-cobalt (Sm-Co) as a main component may be used.

The coercive force of the first to third magnets constituting the magnetic circuit 1 is preferably 650kA/m or more, more preferably 660kA/m or more, still more preferably 700kA/m, and particularly preferably 750 kA/m. When the coercive force is low, the second magnet 12 is likely to irreversibly demagnetize because the operating point is brought close to the H axis by a strong magnetic field generated by the interaction between the first magnet 11 and the third magnet 13. Further, as the coercive force is larger, the magnetic circuit 1 stabilized at a high temperature can be obtained, but the coercive force obtained by the samarium-cobalt magnet is actually 1000kA/m as an upper limit.

The coercive force of the first magnet 11 and the coercive force of the third magnet 13 are preferably equal to each other. This makes it possible to provide a uniform magnetic field to the second magnet 12. However, the coercive force of the first magnet 11 and the coercive force of the third magnet 13 may not be equal.

The second magnet 12 has a coercive force equal to or higher than that of the first and third magnets. Specifically, the coercive force of the second magnet 12 is 1 time or more, preferably 1.05 times or more, and particularly preferably 1.1 times or more of the coercive force of the first and third magnets. Accordingly, even if a strong magnetic field is generated in the vicinity of the through-hole of the second magnet 12 due to the interaction between the first magnet 11 and the third magnet 13, the operating point of the second magnet 12 is less likely to exceed the turning point on the B-H curve or the J-H curve, and the occurrence of irreversible demagnetization due to temperature rise and the inverse magnetic field can be suppressed, and the magnetic flux density in the through-hole portion of the second magnet 12 can be easily maintained to a large extent. In addition, the coercive force obtained from the samarium-cobalt magnet is actually about 400 to 1000 kA/m. Therefore, the coercive force of the second magnet 12 is preferably 2.5 times or less, more preferably 2 times or less, and particularly preferably 1.8 times or less the coercive force of the maximum first and third magnets. When the coercive force of the first magnet 11 and the coercive force of the third magnet 13 are not equal to each other, the magnet having the higher coercive force of the first and third magnets is set to the value described above.

The remanence (Br) of the first to third magnets constituting the magnetic circuit 1 is preferably 0.7T or more, more preferably 0.8T or more, and particularly preferably 0.9T or more. Accordingly, a region having a large magnetic flux density can be formed in the vicinity of the through hole of the second magnet 12, and a rotation angle of 45 ° can be provided to the faraday element 14 described later.

The first magnet 11 and the third magnet 13 preferably have the same residual magnetic flux density. This makes it possible to provide a uniform magnetic field to the second magnet 12. However, the coercive force of the first magnet 11 and the residual magnetic flux density of the third magnet 13 may not be equal to each other.

In the magnetic circuit 1 of the present invention, the length of the second magnet 12 in the optical axis direction is preferably equal to or greater than the length of the first magnet 11 or the third magnet 13 in the optical axis direction. Specifically, the length of the second magnet 12 is preferably 1 time or more, more preferably 1.01 times or more, and particularly preferably 1.05 times or more, the length of the first magnet 11 and the third magnet 13. Accordingly, the length of the magnetization direction of the second magnet 12 is relatively increased, and the magnetic permeability of the second magnet 12 is increased, so that the operating point of the second magnet 12 approaches the B axis side, and the effect of suppressing irreversible demagnetization is increased. Further, when the length of the second magnet 12 in the optical axis direction is too large, the interaction between the first magnet 11 and the third magnet 13 is weakened, and therefore, a region having a large magnetic flux density cannot be formed in the vicinity of the through hole of the second magnet 12. Therefore, the length of the second magnet 12 in the optical axis direction is preferably 2 times or less, more preferably 1.5 times or less, and particularly preferably 1.4 times or less. In addition, in the case where the length of the first magnet 11 in the optical axis direction and the length of the third magnet 13 in the optical axis direction are not equal, the magnet having the longer length in the optical axis direction out of the first and third magnets is made to have the above-described value.

in the magnetic circuit 1 of the present invention, the length of the first magnet 11 in the optical axis direction and the length of the third magnet 13 in the optical axis direction are preferably equal to each other. This makes it possible to provide a uniform magnetic field to the second magnet 12. However, the length of the first magnet 11 in the optical axis direction and the length of the third magnet 13 in the optical axis direction may not be equal to each other.

In the magnetic circuit 1 of the present invention, the cross-sectional shape of the through-hole 2 of the magnetic circuit is not particularly limited, and may be rectangular or circular. A rectangular shape is preferable at a point where assembly is easy, and a circular shape is preferable at a point where a uniform magnetic field is applied.

The cross-sectional area of the through-hole 2 of the magnetic circuit is preferably 100mm ² or less, more preferably 3mm ² to 80mm ², still more preferably 5mm ² to 60mm ², and particularly preferably 7mm ² to 50mm ². when the cross-sectional area is too large, a sufficient magnetic flux density cannot be obtained, and when it is too small, it is difficult to dispose the faraday element 14 in the through-hole 2 of the magnetic circuit.

Fig. 2 is a diagram showing an example of the structure of the first magnet. The first magnet 11 shown in fig. 2 is configured by combining 4 magnet pieces. The number of magnet pieces constituting the first magnet 11 is not limited to the above. For example, the first magnet 11 may be configured by combining 6 or 8 magnet pieces or the like. By combining a plurality of magnet pieces to form the first magnet 11, the magnetic field can be effectively increased. However, the first magnet 11 may be formed of a single magnet.

Fig. 3 is a diagram showing an example of the structure of the second magnet. The second magnet 12 shown in fig. 3 is constituted by 1 single magnet. The second magnet 12 may be configured by combining 2 or more magnet pieces.

Fig. 4 is a diagram showing an example of the structure of the third magnet. The third magnet 13 shown in fig. 4 is configured by combining 4 magnet pieces, similarly to the first magnet 11. By combining a plurality of magnet pieces to form the third magnet 13, the magnetic field can be effectively increased. The third magnet 13 may be formed by combining 6 or 8 magnet pieces or the like, or may be formed by a single magnet.

(Faraday rotator 10)

FIG. 5 is a schematic cross-sectional view showing an example of the structure of a Faraday rotator of the present invention. The faraday rotator 10 is used for a magneto-optical element 20 described later, such as an optical isolator or an optical circulator. The faraday rotator 10 includes a magnetic circuit 1 and a faraday element 14 disposed in a through hole 2 of the magnetic circuit. The faraday element 14 is made of a paramagnetic material that can transmit light.

since the faraday rotator 10 has the magnetic circuit 1 of the present invention shown in fig. 1, irreversible demagnetization caused by an external magnetic field and a temperature rise can be suppressed, and a sufficient magnetic flux density can be stably supplied to the faraday element 14, and thus, the faraday rotator can be stably used.

Further, light may be made incident on the faraday rotator 10 from the first magnet 11 side or may be made incident from the third magnet 13 side.

The cross-sectional shape of the faraday element 14 and the cross-sectional shape of the through hole 2 of the magnetic circuit do not have to be uniform, but are preferably uniform from the viewpoint of providing a uniform magnetic field.

A paramagnetic material can be used for the faraday element 14. Among them, a glass material is preferably used. The faraday element 14 made of a glass material is less likely to have a fluctuation in the verdet constant and a decrease in the extinction ratio due to defects in a single crystal material, etc., and is less likely to be affected by stress from a binder, so that a stable verdet constant and a high extinction ratio can be maintained.

The content of Tb ₂ O ₃ in terms of mole% oxide of the glass material used for the faraday element 14 is preferably more than 40%, more preferably 45% or more, further preferably 48% or more, and particularly preferably 51% or more, so that a good faraday effect can be easily obtained by increasing the content of Tb ₂ O ₃, and Tb is present in a state of 3 or 4 valences in the glass, but all of them are expressed as Tb ₂ O ₃ in this specification.

In the glass material used for the faraday element 14, the ratio of Tb ³⁺ to total Tb is preferably 55% or more, more preferably 60% or more, still more preferably 80% or more, and particularly preferably 90% or more in mol%, and when the ratio of Tb ³⁺ to total Tb is too small, the light transmittance at a wavelength of 300nm to 1100nm is likely to decrease.

(magneto-optical element 20)

Fig. 6 is a schematic cross-sectional view showing an example of the structure of the magneto-optical element of the present invention. The magneto-optical element 20 shown in fig. 6 is an optical isolator. The magneto-optical element 20 includes the faraday rotator 10 shown in fig. 5, a first optical member 25 disposed at one end in the optical axis direction of the magnetic circuit 1, and a second optical member 26 disposed at the other end. The first optical member 25 and the second optical member 26 are polarizers in the present embodiment. The light transmission axis of the second optical member 26 is inclined at 45 ° with respect to the light transmission axis of the first optical member 25.

The light incident on the magnetic optical element 20 is linearly polarized by the first optical member 25 and is incident on the faraday element 14. The incident light is rotated by 45 ° by the faraday element 14 and passes through the second optical member 26. A part of the light passing through the second optical member 26 becomes reflected return light, and passes through the second optical member 26 at an angle of 45 ° to the plane of polarization. The reflected return light having passed through the second optical member 26 is further rotated by 45 ° by the faraday element 14, and becomes a cross polarization plane of 90 ° with respect to the light transmission axis of the first optical member 25. Therefore, the reflected return light is not transmitted through the first optical member 25 and is blocked.

since the magneto-optical element 20 of the present invention has the magnetic circuit 1 of the present invention shown in fig. 1, irreversible demagnetization caused by an external magnetic field and a temperature rise can be suppressed, and a sufficient magnetic flux density can be stably supplied to the faraday element 14, and therefore, the magneto-optical element can be stably used.

the magneto-optical element 20 shown in fig. 6 is an optical isolator, but the magneto-optical element 20 may be an optical circulator. In this case, the first optical member 25 and the second optical member 26 may be wavelength plates or beam splitters. However, the magneto-optical element 20 is not limited to the optical isolator and the optical circulator.